474 



WILSON. 



PLATE XVII. 



[All of the figures from preparations.] 



Fig. 47. Postero-ventral view, thirty-six-celled stage. Origin of the second soma- 

 toblast (6 h. 15 m.). 



Fig. 48. Succeeding stage, thirty-eight cells {cf. Fig. 30) (5 h. 15 m.). 



Fig. 49. Postero-ventral view. Fission of the second somatoblast to form the 

 primary mesoblasts. Transition from Figs. 31 to 32 (6 h. 15 m.). 



Fig. 50. Rear view of same stage as Fig. 37. Fission of X (fourth division), 

 division oi d^, c^, x^; second bilateral division oi c^, d^ (6 h. 15 m.). 



Fig, 51. Ventral view immediately after the fission of X {cf. Fig. 38) (7 h.). 



Fig. 52. Rear view. Fifth division of X, x^ dividing, x^ divided into two (6 h.). 



Fig. 53. The same stage, seen more from the ventral side; x^ undivided, x^ divid- 

 ing. Fifth division of X. First gemmation of primary mesoblasts (6 h.). 



Fig. 54. Postero-ventral view. Sixth division of X (r/i Figs. 42, 43) (7 h. 30 m.). 



Fig. 55. Ventral view of later stage. Four secondary mesoblast cells have been 

 formed, the primary mesoblasts are nearly covered in, and the blastopore is nearly 

 closed (9 h.). 



Fig. 56. Slightly later stage. Eighth division of X (fission of the proteloblasts), 

 gemmation of the primary mesoblasts (probably the third), approximation of the 

 stomatoblasts, division of ^^ {cf. Figs. 45, 46) (9 h. 45 m.). 



Fig. 57. Similar view of later stage (the same specimen shown in Fig. 74) 

 (9 h. 30 m.). 



Fig. 58. Rear view of still later stage, showing the four posterior teloblasts 

 (8 h. 30 m.). 



